Ray - keep us posted as you test. Will be interesting to see your PCR numbers drop from current levels while taking 50 mg dasatinib.
Testing monthly during your transition from imatinib to dasatninib is wise. Also - should your PCR plummet - (I have a suspicion it will), consider lowering dose further to 20 mg. You will minimize side effects - including side effects you do not necessarily feel and you will keep your immune system stronger as well. You can always increase dose should PCR not fall.
In my case, my doctor did not gradually drop my dose. He started me at 70 mg (never 100) which only lasted a week. When I restarted dasatnib, he restarted me at 20 mg. because of his knowledge and experience with the drug. My PCR levels fell dramatically on that dose and I have been at that dose ever since (3 years 'undetected'). Now I take dasatnib 20 mg every other day weaning myself off it in preparation for complete cessation.
One other note - in terms of progression free survival or dying from CML. The gold standard is CCyR - complete cytogenetic remission. CCyR is zero CML cells observed under the microscope which corresponds approximately to 1.0% PCR. As long as a patient is CCyR, they have the same progression free survival statistics as patients in MMR (or undetected).